In many operations, equipment for processing raw materials received from a conveyor belt can be damaged when these materials are contaminated with metal objects. Prior to this invention, removal of metal objects from a moving belt was accomplished by stopping the belt and searching its overburden manually. The search was usually conducted immediately downstream of a metal detector which had signaled the presence of a metal object traveling on the belt. Only when the metal object had been found could operations be resumed. To reduce the time the conveyor belt might need to be shut down, personnel were employed to stand by and to be ready to search, at a moment's notice, for a metal object. This approach proved to be costly not only in terms of manpower but also in terms of production losses. Moreover, in situations in which two or more metal objects were traveling in close proximity to each other on the belt, the metal detector might indicate the presence of only one of these objects.
A mechanism for removing certain undesirable objects from a moving conveyor belt without stopping it is disclosed by Wilson in U.S. Pat. No. 5,018,619, issued May 28, 1991. Wilson's mechanism comprises an assembly of horizontally disposed idlers of varying lengths. The idlers in this assembly are so arrayed that they support a portion of the belt across substantially less than its entire width, thereby creating a soft spot in the belt. When oversized, but not necessarily metallic, objects traveling on the belt pass over the soft spot, the belt forms a chute, allowing the oversized objects to slide laterally and off of the belt. In the cited patent, separators are employed which selectively deflect oversized objects sideways and onto the soft spot; but these separators cannot distinguish between metallic and non-metallic objects.